Transmission oil coolers of the type that are carried within one of the radiator tanks include at least a pair of generally rectangular plates between which is captured a cooling fin of some sort. The plates define an oil inlet at one end and outlet at the other, so that the oil flow is generally longitudinal. The cooling fin, around and through which the hot oil flows, conducts heat out to the plates which, in ram, are continually bathed in the cooler radiator coolant. Because of the hot and corrosive environment; the fin is made from steel, as opposed to the softer copper and aluminum generally found on the air side of radiators, condensers and the like. Typically, the shape of such steel oil cooler fins was a "lace curtain" turbulator, which, unlike the corrugated "air center" found in radiators, did not have distinct fin walls as such. Lately, however, it has been found that the incorporation of a true, corrugated cooling fin, with fin walls joined to one another in a series of V shapes, can significantly improve performance. As disclosed in co-assigned U.S. Pat. No. 4,945,981 issued Aug. 7, 1990 to Joshi, a corrugated fin is captured between the plates, embodied either as a fin with shorter fin walls extending across the width of the plates (transverse to the axis of the plates), or longer fin walls running parallel the axis of the plates. The transverse fin wall embodiment has been used in actual production, and its fin walls are pierced by sharp angle louvers to allow the oil to flow end to end of the cooler. Otherwise, the transverse running fin walls would block oil flow.
The transverse fin wall embodiment from the above patent has presented a manufacturing problem not evident from the disclosure therein, or, more accurately, a problem in achieving optimal thermal efficiency within the limits of manufacturability. The drawings in the patent, which was concerned only with the shape of the fin, do not accurately reflect the actual shape of the plates, at least the shape near the marginal edges of the plates. As seen in FIGS. 1 and 2 of the drawings, which accurately depicts the shape of an actual production embodiment, oil cooler 10 has at least two plates, (an actual cooler could include a number of plate pairs stacked together), a lower or "male" plate 12 and an upper or "female" plate 14. The plates are so denominated because the male plate 12 is slightly narrower (and slightly shorter) so as to fit inside the female plate 14 all the way around to create a continuous overlapped seam around the margin, at which the two are brazed together. One or both plates is ported at each end so that oil flows along and between the plates 12 and 14, as shown by the arrows, along the longitudinal axis A. Although each is symmetrical about its axis and generally rectangular, neither plate 12 nor 14 is sharp cornered at the ends, but instead rounded off into a semi-circular end shape. Furthermore, neither has a sharp, fight angled comer at the margins. Instead, as best seen in FIGS. 2 and 3, each makes a more rounded transition into its marginal edge, all the way around. For example, upper plate 14 has a central flat area 16, the width of which comprises most of its total width W.sub.1. The total width W.sub.1 will vary over particular designs. Outboard of the flat area 16, lower plate 12 makes a gradual, generally semi-cylindrical transition into its marginal edge 18, over a distance of about 0.075 inches, which, while narrow, is still a significant percentage of its total width. Likewise, lower plate 12, which has a slightly smaller width W.sub.2, just enough smaller than W.sub.1 to accommodate the upper plate 14 being interfitted snug over its marginal edge 18). Lower plate 12 makes the same semi-cylindrical transition from a central flat area 20 (which has nearly the same width as flat area 16) down to marginal edge 22, and over approximately the same distance.
The reason for the semi-cylindrical shape of the transition areas along the margins of the plates 12 and 14, besides avoiding stress concentrations at sharp corners, is to allow the two plates to be crimped together, as shown in FIGS. 2 and 3. A pair of clinch dies, not illustrated, pushes the two plates together, and the upper clinch dies progressively bends the initially straight marginal edge 22 of upper plate 14 around and over the marginal edge 22 of lower plate 12 to create a smooth, snug overlapped seam, clearly seen in FIG. 3. Another consequence of the rounded comers of the plates 12 and 14 is not so obvious. The corrugated steel fin 24 that is captured between the plates 12 and 14 has corrugations that run transverse to the axis A. It has an effective fin height H which is substantially equal to the axial separation or thickness T, which is about 0.133 inches as disclosed. The term "effective fin height" indicates that the fin height acting to separate the plates 12 and 14 is a function of the total, peak to peak fin wall length, as well as the angle of the fin wall. If the fin walls are parallel (zero angle), then the fin wall length and fin height are one and the same. Given the strong material from which the fin 24 is formed, its corrugations, once captured and confined between the plates 12 and 14, are very stiff. Therefore, the total, side edge to side edge width of fin 24 transverse to the axis A cannot be made significantly greater than the corresponding width of the plate central flat areas 16 and 20. Otherwise, the side edges of fin 24 would intrude out into the semi cylindrical transition areas between the plates 12 and 14 located outboard of the flat areas 16 and 20. If the intrusion were too great, the stiffness of fin 24 could overpower the clinch dies, jeopardizing the integrity of the seam formed between the overlapping marginal edges 18 and 22. As a consequence, a bypass area indicated at B is left open, which borders both sides of fin 24, running end to end of the cooler 10. The bypass area B is essentially a half round pipe with a radius (or width) equal to the distance over which the plate flat areas 16, 20 and transition into the marginal edges 18, 22, or about 0.075 inches. The width of the bypass area B is a significant percentage of the total width of the cooler 10. Since by pass area B presents a much lower resistance to flow than the louvered fin 24 itself, a significant volume of oil can flow through it, and not through the fin 24, thereby reducing the potential thermal efficiency of oil cooler 10. There is no obvious way to extend the width of a fin like 24 out into the bypass area B without jeopardizing its manufacturability.